1. Field of the Invention
This invention relates to a method of treating or decontaminating waste photographic processing solutions containing sulfur compounds of silver and a method of recovering silver using chemo-synthetic sulfur bacteria, and in particular, to a novel method by which the oxidation of thiosulfate ion, i.e., environmental decontamination of the wastes and silver recovery through accumulation in bacteria cells are accomplished either simultaneously or succesively depending on the purposes.
2. Description of the Prior Art
Of various processing solutions for silver halide photography now employed in practical use, the developer solution and the fixer solution are the most fundamental. The fixer solution, which is employed to dissolve and remove silver halide remaining in the unexposed areas after development, comprises, in addition to a high concentration of thiosulfate salt as a principal agent, sodium sulfite, acetic acid, potassium bromide, etc. The fixer used for processing and which has become exhausted contains, as a sulfur compound of silver, solubilized silver ion in the form of silver thiosulfate complexes, and the other components described above. Hence, the waste fixer solution has very high values for various indicators of the degree of environmental contamination, such as BOD (biological oxygen demand), COD (chemical oxygen demand) and TOD (total oxygen demand). Decontamination or treatment to drastically reduce these oxygen demand values is required. The activated-sludge process is widely used to decontaminate waste waters which contain various substances. But, unfortunately among the various components, the thiosulfate salt, which is present in the largest concentration in the waste fixer solution cannot easily be oxidized using a conventional activated-sludge treatment (e.g., as disclosed in Kodak Publication Nos. J-28, J-41 and J-46, Eastman Kodak Company, Rochester N.Y. 14650), and an efficient treatment needs to be developed.
On the other hand, silver which is present in the waste fixer solution has been recovered using various methods including electrolysis (e.g., as disclosed in Kodak Publication No. J-10, Eastman Kodak Company, Rochester N.Y. 14650). However, when the concentration of silver is about 100 mg/l or less, no attempts have been made for a practical recovery. Accordingly, concentration of silver to such a degree that easy recovery is possible is desirable from the viewpoint of conservation of silver as a valuable resource.